JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = {
    "localValues": [{
        "localValId": "8KfbEozjdQYAefuHF",
            "localProductCode": "291105300",
            "localMarkupVal": "2.8",
            "localMembersPrice": "3344"
    }, {
        "localValId": "qdCY6Kkvc7e8m4yxw",
            "localProductCode": "291105300234",
            "localMarkupVal": "2.8432",
            "localMembersPrice": "3344333"
    }, {
        "localValId": "i827Eve8zBexRSTSP",
            "localProductCode": "291105300",
            "localMarkupVal": "2.8432",
            "localMembersPrice": "899"
    }]
}

    function findValue(data, key, value) {
        return data.map(function (el, i) {
            return el[key] === value
        }).map(function (el, i) {
            return el === true ? i : null
        }).filter(function (el) {
            return el !== null;
        });
    }

console.log(findValue(data.localValues, 'localProductCode', '291105300'));